The present invention pertains generally to a method for repairing mask defects and in particular to a method for repairing amplitude and/or phase defects on mask blanks by optical compensation.
Photolithography is a well known technique for applying patterns to the surface of a workpiece, such as a circuit pattern to a semiconductor chip or wafer. This technique has additional advantage in being able to faithfully reproduce small and intricate patterns. Traditional photolithography involves applying electromagnetic radiation to a mask having openings formed therein (transmission mask) such that the light or radiation that passes through the openings is applied to a region on the surface of the workpiece that is coated with a radiation-sensitive substance, e.g., a photoresist. The mask pattern is reproduced on the surface of the workpiece by removing the exposed or unexposed photoresist.
However, the capabilities of conventional photolithographic techniques have been severely challenged by the need for circuitry of increasing density and higher resolution features. The demand for smaller feature sizes has not only driven the wavelength of radiation needed to produce the desired pattern to ever shorter wavelengths but has also lead to improvements in transmission (amplitude-only) masks that often involve phase shifting techniques in which certain of the openings, or portions of openings, are phase shifted with respect to adjacent openings.
The trend toward shorter wavelengths has also required the development of a different type of mask because of the fact that extreme ultraviolet radiation (EUV), i.e., radiation in the spectral region between 10 and 15 nm, is strongly absorbed generally by condensed matter. Thus, efficient transmission of EUV radiation requires the use of reflective optics. Consequently, a mask useful for photolithography employing EUV has a unique architecture, shown in FIG. 1, consisting of a reflective multilayer coating deposited onto a highly polished defect-free substrate. A patterned absorber layer is disposed on the surface of the reflective multilayer coating. The multilayer, required to make the substrate reflective at EUV wavelengths) is composed of alternating layers of EUV-reflective material; a typical multilayer coating, or stack, can be a 40-layer pair of metallic Mo and Si with a periodicity of about 7 nm. The mask substrate having a multilayer stack disposed thereon is commonly referred to as the mask blank. Phase-shifting methods can be employed by changing the material of the absorber from a purely opaque type to a partially transmitting material that induces a phase shift. Germanium is an example of a phase-shifting absorber material that can be used for EUV lithography.
For all lithographic technologies there are certain classes of defects that are difficult if not impossible to repair directly. In the case of conventional lithographic technologies employing transmission phase shift masks, repair not only requires making clear areas totally opaque but also opaque areas totally clear and, in addition, repairing phase defects. A phase defect is a defect wherein the mask transmits radiation where it should but of the wrong phase. The intensity as well as the phase of the repaired area must be within an acceptable tolerance to preserve the desirable imaging performance of a perfect mask. Because of the importance of phase shifting masks, considerable effort has been spent in trying to overcome problems of defect repair. As set forth in U.S. Pat. No. 5,795,685 entitled "Simple Repair Method for Phase Shifting Masks" issued to Liebman et al. on Aug. 18, 1998, one solution to the problem of repairing damaged phase shift masks involves identifying and mapping the defective region of a first phase shift mask, making it opaque, then making a phase shift repair mask that operatively cooperates with the first mask as well as functioning as a "trim" mask. While the approach in the '685 patent does not offer a solution to the difficult problem of repairing phase shift masks, it requires making a second mask which can be expensive and time consuming.
While defects in the absorber layer of a reflective mask can be repaired by art-recognized methods, such as a focused ion beam, flaws or defects in reflective mask blanks that alter the magnitude or phase of reflected EUV optical fields and that print in the lithographic process are, at present, impossible to correct. Defects are considered printable if their presence affects the process window of nearby features by more than 10%. The reason these defects cannot be corrected can be readily seen by referring to FIG. 1. A defect disposed on a Si or glass substrate surface or within the multilayer stack will be replicated throughout the multilayer reflector. Because the entire multilayer region acts to reflect EUV radiation, the only way to eliminate the effect of these defects is to remove and redeposit the entire multilayer in the vicinity of the defect, an impossible task. Additional sources of defects can include substrate imperfections such as dislocations and stacking faults that are replicated or partially replicated in the multilayer coating process.
What is needed is a simple method for repair of phase shift masks as well as a method of repairing printable defects in reflective masks that are presently impossible to repair.